1. Field of the Invention
The present invention relates to an engine part, and more particularly, the present invention relates to an engine part which is subjected to a high temperature due to a high-temperature exhaust gas discharged from an engine.
2. Description of the Related Art
In a vehicle such as a motorcycle or an all-terrain four-wheeled vehicle, not only the performance of the vehicle itself, but a good vehicle design are vital. FIG. 10 is a side view showing an example of a sports-type motorcycle. A motorcycle 200 shown in FIG. 10 includes a V-type engine 201 and an exhaust pipe 202 for guiding along exhaust gas. The V-type engine 201 includes cylinders 203, cylinder heads 204, and head covers 205. The aesthetically excellent V-type engine 201 is likely to be mounted to the motorcycle such that the engine is exposed on the outside, and is highly influential to the exterior appearance of the entire motorcycle.
The two cylinders 203 of the V-type engine 201 are united at the single exhaust pipe 202, which extends toward and above the rear wheel so as to allow exhaust gas to be discharged at the rear portion of the body. The exhaust pipe 202 must have a certain thickness for allowing the exhaust gas generated in the engine 201 to be efficiently discharged. Moreover, the portion constituting a muffler 202a has an increased diameter in order to accommodate the muffling structure. For these reasons, the exhaust pipe accounts for a relatively large part of the exterior appearance of the entire motorcycle, and thus the shape and color of the exhaust pipe are highly influential to the entire motorcycle design.
In the present specification, engine components such as the cylinders 203, the cylinder heads 204, the head covers 205, as well as the exhaust pipe 202 for guiding the exhaust gas from the engine, will be generally referred to as “engine parts”. For the aforementioned reasons, the shape and color of engine parts are important factors in determining the entire motorcycle design.
Conventionally, those engine parts which show up on the exterior appearance are subjected to surface treatments such as plating to acquire a lustrous metallic color, thus to enhance the engine part design. Above all, decorative chromium plating has been widely used for engine parts because it is possible to give the plated material a characteristic, lustrous silver-gray color (see, for example, Japanese Laid-Open Patent Publication No. 2003-41933).
Since decorative chromium plating provides an excellent metallic luster, and also excels in anticorrosiveness, it is also used in various fields other than engine parts. In order to obtain excellent exterior appearance and anticorrosiveness, it is unnecessary to provide a thick layer of decorative chromium plating. In fact, a thick layer of decorative chromium plating will result in a poor color tone and surface finish. Therefore, in general, decorative chromium plating is likely to be used at a thickness in the range from 0.1 μm to 0.15 μm.
On the other hand, hard chromium plating (industrial chromium plating) is also widely used as Cr plating in industrial products. Since hard chromium plating provides a low friction coefficient and an excellent abrasion resistance, it is used for sliding sections of various machine parts, for example. Since abrasion resistance is a requirement, hard chromium plating is usually formed to a thickness of at least several μm. Moreover, hard chromium plating does not provide a decorativeness surface as does decorative chromium plating. Generally, decorative chromium plating provides a surface roughness (Ra) of about 1 μm or less (typically, about 0.2 μm or less), while hard chromium plating provides a surface roughness of more than about 1 μm.
In such engine parts, due to the high-temperature exhaust gas generated from the engine, the surface of the chromium plating layer may change its color tone to result in a violet discoloration, or the decorative chromium plating layer may have cracks and then peel. Particularly in recent years, the engine performance has improved, and catalysts have been used for exhaust gas purification, and as a result the exhaust gas temperature has increased. Thus, engine parts are becoming more susceptible to discoloration due to high-temperature exhaust gas. As described above, engine parts account for a relatively large portion of the overall exterior appearance of the entire motorcycle. Therefore, even a muddy spot on the chromium plating may greatly impair the entire design.
This problem may be addressed by preventing the temperature of the surface of the exhaust pipe or the like from becoming too high, by adopting a two-fold or three-fold cylindrical structure for the exhaust pipe, for example. However, even by adopting a two-fold or three-fold cylindrical structure, the temperature of the exhaust pipe surface will not be adequately lowered, and surface oxidation or deterioration due to heat will not be completely prevented. Moreover, in this case, there is another problem in that the outer dimensions of the exhaust pipe or the like are increased.
It might be possible to prevent discoloration or deterioration of an engine part surface by covering the exhaust pipe (for example) with a cowl or a protector so that the exhaust pipe will not show on the exterior. However, in this case, the exhaust pipe no longer contributes to the entire motorcycle design, thus making it difficult to pursue the characteristic beauty of a motorcycle. In particular, covering an exhaust pipe with a protector will result in a diameter which is greater than the exhaust pipe itself, thus worsening layout constraints.
A decorative chromium plating layer is usually formed by using a chromate, including hexavalent chromium (Cr6+). Hexavalent chromium is inexpensive. A decorative chromium plating layer obtained by using hexavalent chromium shows good contact with a base substrate, and has excellent anticorrosiveness and abrasion resistance. A chromium plating layer obtained by using hexavalent chromium has a silver-gray color with a characteristically metallic luster. Therefore, hexavalent chromium is widely used in engine parts for motorcycles, for example. However, its toxic nature has been recognized in the recent years. Hence, proposals have been made to use trivalent chromium (Cr3+), instead of hexavalent chromium, to obtain decorative chromium plating. Although trivalent chromium is inferior to hexavalent chromium in terms of anticorrosiveness, contact with the base substrate, and the like, environmental pollution concerns and safety-oriented thinking have led to the trends toward selective use of trivalent chromium. The trend for such alternative plating, i.e., trivalent chromium replacing hexavalent chromium, is also becoming popular in the field of engine parts. For example, a motorcycle having a trivalent chromium-plated protector provided on the outer periphery of a muffler has recently been developed.
However, according to the studies by the inventor, discoloration of the plating layer and surface deterioration due to a high-temperature heating are observed even in the case where trivalent chromium is used, and in fact, such discoloration and deterioration are more noticeable than in the case where hexavalent chromium is used.
Furthermore, a decorative chromium plating layer obtained by using trivalent chromium has a slightly blackish color tone. This presents a new problem in that the silver-gray color tone obtainable by using hexavalent chromium is difficult to obtain. Such a difference in color tone can be highly problematic in the field of motorcycles and the like, where exterior appearance is regarded as important. Therefore, there is a desire for engine parts which, even by using trivalent chromium, attain a similar color tone to that which is obtained by using hexavalent chromium.